The spraying method is a technique that a coating is formed on a surface of a substrate by melting powder off a metal, a ceramic, a cermet or the like with a plasma or burning flame and then blowing onto the surface of the substrate (a body to be sprayed) under flying, which is one of surface treating techniques widely adopted in many industrial fields. However, the spray coating obtained by laminating fine particles at a molten state is known to cause a large difference in the mechanical strength and corrosion resistance of the coating in accordance with strong and weak bonding force of the particles constituting the coating, amount of particles not bonded and the presence of non-molten particles. For this end, the conventional target of developing the spraying technique lies in that a large moving energy is given to the sprayed particles by using an apparatus for realizing the complete melting of sprayed particles using a high temperature heat source, for example, a plasma jet flame as a heat source, or a high-velocity combustion flame to generate a strong impact energy on the surface of the substrate to be sprayed, whereby the bonding force between the particles is enhanced and the porosity is reduced and further the joining force between the coating and the substrate is improved.
For example, JP-A-1-139749 proposes a method wherein the bonding force between the particles is improved or the oxide film produced on the surface of the metal particle, which is a cause of generating pores, is reduced by adopting a method of spraying a plasma in an argon gas atmosphere of 50-200 hPa.
Recently, the characteristics of the spray coating are improved by the above proposal. However, there is not much the examination on the color of the coating formed by using the same spraying material. As the color of the ceramic spray coating is observed, chromium oxide (Cr2O3) powder as a spraying material is deep green near to black, but when it is plasma-sprayed, a black coating is obtained. On the other hand, aluminum oxide (Al2O3) powder is white and also a coating obtained by plasma-spraying is white. On the contrary, titanium oxide (TiO2) powder is whitish, but when it is plasma-sprayed, a blackish coating is obtained. Thus, a cause on the color change of the spray coating is considered due to the fact that a part of oxygen constituting TiO2 disappears in the heat source for spraying to form an oxide shown by TinO2n-1 (see JP-A-2000-054802).
As mentioned above, it is general that as to the color of the oxide-based ceramic spray coating getting off a part of oxides, the color of the powder material for spraying itself is reproduced as the color of the coating as it is. For example, yttrium oxide (Y2O3) usually shows a white color at not only a state of powder material but also a state of a spray coating obtained by spraying likewise Al2O3. It is considered that even if Y2O3 is sprayed in a plasma heat source, there is no change in the bonding state between Y and O (oxygen) constituting Y2O3 particles. That is, it is considered that since Al and Y as a metallic element are very strong in the chemical affinity with oxygen, the oxide does not disappear oxygen even in a high temperature plasma environment, and even after the formation of the spray coating, the characteristics of Al2O3 or Y2O3 are maintained as they are.
The Y2O3 spray coating is excellent in the heat resistance, resistance to high-temperature oxidation and corrosion resistance and develops a remarkable resistance even in a plasma etching environment used in a semiconductor producing apparatus or a working step thereof (resistance to plasma erosion), so that it is a ceramic coating used in many industrial fields (JP-A-6-196421, JP-A-10-004083, JP-A-10-163180, JP 2001-31484A, JP-A-2001-164354).
All of the Y2O3 spray coatings usually used show a white color and develop some positive effects, but there is no proposal on the change of color in the coating without changing the characteristics of the Y2O3 spray coating.
As a technique of modifying the surface of the substrate, these is a technique of utilizing an electron beam irradiation or a laser beam irradiation in addition to the above formation of the spray coating. As to the electron beam irradiation, for example, there are known a technique of JP-A-61-104062 wherein electron beams are irradiated to a metal coating to melt the coating to thereby remove the pores, and a technique of JP-A-9-316624 wherein electron beams are irradiated to a carbide cermet coating or a metal coating to improve the performances of the coating.
However, these conventional techniques are a technique of targeting the carbide cermet coating and the metal coating and for the purpose of the disappearance of pores in the coating and the improvement of the adhesion property. Particularly, these techniques are not a technique of adjusting the color of ceramic spray coating, particularly Y2O3 spray coating.
Further, these techniques are considered to be due to a fixed concept that an electrically conductive coating is required in the treatment of the spraying material with the electron beams as disclosed on paragraph [0011] of JP-A-9-316624. On the other hand, JP-A-9-327779, JP-A-10-202782 and the like disclose the technique of irradiating the laser beams to the spray coating. However, this technique frequently targets the ceramic coating such as metal coating, carbide cermet and the like. In this technique, however, even if the underground is the ceramic coating, the object of the treatment is to disappear the pores of the coating or to promote the occurrence of longitudinal crack utilizing a phenomenon that the coating shrinks in the cooling step after the melting of the coating, and the ceramic coating to be targeted is ZrO2 system.